Designs and methods for conductive bumps

ABSTRACT

Methods, techniques, and structures relating to die packaging. In one exemplary implementation, a die package interconnect structure includes a semiconductor substrate and a first conducting layer in contact with the semiconductor substrate. The first conducting layer may include a base layer metal. The base layer metal may include Cu. The exemplary implementation may also include a diffusion barrier in contact with the first conducting layer and a wetting layer on top of the diffusion barrier. A bump layer may reside on top of the wetting layer, in which the bump layer may include Sn, and Sn may be electroplated. The diffusion barrier may be electroless and may be adapted to prevent Cu and Sn from diffusing through the diffusion barrier. Furthermore, the diffusion barrier may be further adapted to suppress a whisker-type formation in the bump layer.

This is a Continuation of application Ser. No. 12/973,615 file Dec. 20,2010 which is a Continuation of application Ser. No.: 11/894,627, filedAug. 20, 2007 now U.S. Pat. No. 8,580,679 issued Nov. 12, 2013 which isa Divisional application of Ser. No. 10/668,986, filed Sep. 22, 2003 nowU.S. Pat. No. 7,276,801, Issued on Oct. 2, 2007.

TECHNICAL FIELD

This specification relates to semiconductor process fabrication, andmore particularly to fabricating bumps for integrated circuits, as indie packaging.

BACKGROUND

During a die packaging process, several conductive layers may be placedbetween the substrate of a die and the surrounding package. The diepackage can be soldered with a conductive layer and the soldered layermay contact a lower-level conducting layer. The lower-level conductinglayer may be patterned to have one or more conducting bumps, and may bereferred to as a “bump” layer. A bump may contact a base layer metal(BLM) that is directly or indirectly connected to the substrate. Thebump and the base layer metal may have one or more properties that mayresult in one or more electromigration issues or degradation of thelayers.

DESCRIPTION OF DRAWINGS

FIGS. 1A-1D show exemplary defective bump diagrams.

FIG. 2 shows an exemplary implementation of the fabricated structure.

FIG. 3 is an exemplary flow diagram of the fabrication of the exemplaryimplementation shown in FIG. 2.

FIG. 4 shows an exemplary implementation of the fabricated structure.

FIG. 5 is an exemplary flow diagram of the fabrication of the exemplaryimplementation shown in FIG. 4.

FIG. 6 shows an exemplary implementation of the fabricated structure.

FIG. 7 is an exemplary flow diagram of the fabrication of the exemplaryimplementation shown in FIG. 6.

FIG. 8 shows an exemplary implementation of the die package structure inan electrical/computer system.

DETAILED DESCRIPTION

The techniques, methods, and structures of one or more exemplaryimplementations in the present disclosure relate to integrated circuitsand die packaging. In particular, one or more exemplary implementationsrelate to fabricating bumps on a substrate to prevent Cu and Snintermixing. One or more exemplary implementations in the presentdisclosure may reduce a number of electromigration issues relating toCuSn intermetallic formation, and may reduce the formation of whiskersin one or more layers.

The details of one or more exemplary implementations are set forth inthe accompanying drawings and the description below. In one exemplaryimplementation, an apparatus comprises a semiconductor substrate and afirst conducting layer in contact with the semiconductor substrate. Thefirst conducting layer may comprise a base layer metal, such as Cu. Theapparatus further comprises a diffusion barrier in contact with thefirst conducting layer, a wetting layer on top of the diffusion barrier,and a bump layer on top of the wetting layer. The bump layer may includeSn, and the Sn bump layer may be electroplated. The diffusion barriermay prevent Cu and Sn from diffusing through the diffusion barrier. Thediffusion barrier may also be able to suppress a whisker-type formationin the bump layer. Other features and advantages of one or moreexemplary implementations will be apparent from the description anddrawings, and from the claims.

In semiconductor wafer processing, devices and interconnects are formedon a substrate and are electrically connected to a die package. Anelectrical connection to a die package may be achieved with a conductingsolder layer between the die package and a lower-level conductinginterconnect layer on the wafer. The solder layer may oftentimes includeSn or a Sn alloy. The conducting interconnect layer, or an adjacentconducting layer, may oftentimes include Cu. In some cases, theconducting interconnect layer may be the lowest level metal layer or themetal layer that is in closest proximity to the substrate. Such a metallayer may be referred to as a base layer metal (BLM). In some cases, thebase layer metal may be used as a diffusion barrier to prevent solderfrom migrating into a lower-level pad of the die. The pad of the die mayinclude one or more layers of metal, such as an Al layer. In one or moreexemplary implementations of the present disclosure, a layer may beformed on top of the base layer metal to serve as a diffusion barrierbetween Cu in the base layer metal and Sn in a layer above the diffusionbarrier.

A die package interconnect structure with Sn in one layer and Cu in anearby layer may result in one or more detrimental issues for the diepackage interconnect. Some of these detrimental issues may degrade theelectrical and mechanical properties of the die package interconnect,reduce the yield of forming such interconnects, or even form irregular,unintended regions such as whiskers and delaminations. Delamination mayinvolve the degradation or the physical separation of one or morelayers. Some of these detrimental issues are exemplified in FIGS. 1A-1Dand are described below.

FIGS. 1A-1B present diagrams of a die package interconnect with anexemplary bump delamination. The bump 110 in the diagram of FIG. 1A is asolder region that includes a Sn alloy, PbSn. As shown in a markedregion 115, a bump 110 may conform to the shape of the lowerinterconnect or the base layer metal 125. The bump 110 may contact andcover the edges and sidewalls of lower interconnects or the base layermetal.

FIG. 1B is a diagram of the region 115 in which the base layer metal125, the PbSn bump 110, and the die substrate area 130 are presented. Inthis diagram, delamination occurs in the layers 125, 110 between thesubstrate 130 and the bump 110. Such delamination may result from one ormore properties of the base layer metal 125. The base layer metal 125may be constructed in such a manner that the base layer metal 125 maydegrade during baking or thermal processing. Degradation anddelamination may reduce the conducting surface area between the layers125, 110 and create electromigration-related failures.

Some failures in die package interconnects may beelectromigration-related failures and other failures may be due to theproperties of the layer materials and the interfacing of layers. Someelectromigration-related failures may be due to the metallurgicalproperties of the layers 125, 110, increased heat and thermal issues,and the growth of one or more voids 120 between the layers 125, 110.Electromigration in region 115 may create higher current densities andincrease electromagnetic stress. As described below in one or moreexemplary implementations, methods, structures, and techniques arepresented to reduce electromigration-related die package interconnectfailures.

Certain metallurgical properties of one or more layers of die packageinterconnects may result in die package interconnect failure. Examplesof such metallurgical properties include non-conforming surfaces, phasetransitions of materials at different temperatures, and diffusion andintermixing of elements of different layers. For instance, Sn can be acommon metal used in one or more die package interconnect layers.However, Sn may exist in two allotropes at different temperatures. Abovea temperature of about 13.2° C., the hard, shiny, and conductive alphaSn (tetragonal structure, a-phase Sn) may be in a stable phase. When thetemperature is below 13.2° C., beta Sn (diamond cubic structure, β phaseSn) may be thermodynamically favorable. The alpha phase is a preferredphase in a layer structure. The alpha to beta phase transformation maybe accompanied by a 26% volume increase due to different densities oftwo phases. The change in volume in the phase transition may deform theinterface between Sn and other layers. Also, beta Sn is in powder formand does not have the mechanical strength for an interconnect. Hence,when the Sn is in beta phase, the mechanical strength of the Sn layerand the interconnect deteriorates. For at least the above reasons, a Snlayer or interconnect can transition from alpha phase to beta phaseduring low temperatures and may lead to interconnect failure. Asdescribed below in one or more exemplary implementations, methods,techniques, and structures are presented to prevent low temperaturephase transition of Sn.

Although bump 110 is shown as a solder bump in 100, the bump may be abump or a bump layer that is not a solder layer, but a layer adjoining asolder layer. Moreover, the bump 110 may not be directly contacting thedie package 105. As described in the figures below, the bump or bumplayer could include other materials, such as Cu, and may contactdirectly to the base layer metal or other layer interconnects.

FIGS. 1C-1D present examples of the undesirable intermixing of Cu and Snof different layers. For instance, FIG. 1C shows a diagram of a diepackage interconnect 150 with a void 155 in a solder layer. Diffusion orintermixing of Cu and Sn from different layers may form CuSnintermetallics and may help to create the void 155. The electricalresistance in the interconnect 150 may increase due to voiding in thesolder and may result in electromigration problems. Other undesirableformations, such as whiskers, may also form due to the intermixing of Cuand Sn. The whiskers may result in compressive stress build up in Snbumps and may lead to die package interconnect failure.

FIG. 1D shows another exemplary diagram 160 of various layers of a diepackage interconnect with diffused or intermixed Cu and Sn. FIG. 1Dshows examples of Cu and Sn formation. For instance, a region 165 ofCu₃Sn and a region 170 of Cu₆Sn₅ have formed between layers of Cu 172,178 and Sn 176 in the diagram 160.

Some conventional techniques attempt to prevent the intermixing anddiffusion of Cu and Sn from different die package interconnect layers.For instance, the use of Pb₅Sn bumps may be used to prevent whiskerformation. However, the use of Pb5Sn bumps may have electromigrationissues that may result from low temperature phase transition of Sn, asdescribed above. Moreover, Pb may contribute to environmental and healthissues. In another conventional example, sputtered Ni may be used toprevent Cu diffusing into Sn. However, sputtered Ni has poor diffusionbarrier properties and does not adequately prevent Cu and Sn diffusionor intermixing. As described below in one or more exemplaryimplementations, methods, techniques, and structures are presented toprevent the diffusion and intermixing of Cu and Sn between different diepackage interconnect layers.

One or more exemplary implementations in the present disclosure alsopresent methods, techniques, and structures to prevent degradation of Snbumps during etching of the base layer metal. In general, the corrosionand oxidation of the Sn bumps may be prevented during etching of a baselayer metal that includes Ti, Al, or NiV.

FIG. 2 shows an exemplary implementation of a die package interconnect.A base layer metal 230 is formed on top of a silicon substrate 205 andthe base layer metal 230 may include Cu. The base layer metal 230 mayalso be formed on top of a patterned insulator, resin, or dielectriclayer 235, such as a polyimide layer. The base layer metal 230 mayinclude an adhesion layer 232 and a seed layer 234. The adhesion layer232 may be formed, for example, on the substrate 205 or a dielectriclayer 235. The adhesion layer 232 may help in the joining or attachingof two different surfaces, such as helping the base layer metal to stickto the underlying surface. The seed layer 234 may help to build up thebase layer metal structure on top of the adhesion layer 232. The seedlayer may serve as a smooth interface to an underlying layer and mayfacilitate correct growth and formation of the base layer metal. Theadhesion layer 232 may include Ti, TiN, and TiSiN and the seed layer 234may include Ni, NiV, and Co. An additional metal layer, such as an Allayer 233, may be formed between the adhesion layer 232 and the seedlayer 234 in order to improve one or more properties of the base layermetal. The improvement of the base layer metal properties with theadditional metal layer may include the suppression of whisker formationsand the prevention of layer delamination and degradations during thermalprocessing and electromagnetic stress.

A diffusion barrier layer 225 can be selectively positioned on top ofthe base layer metal 230. Selective deposition may mean that somesurfaces may have another layer deposited only on a portion of thatsurface. The electroless diffusion barrier can prevent Cu and Sn fromdiffusing through the diffusion barrier. The diffusion barrier layer 225may be electroless and located in a position to prevent the intermixingof Cu from the base layer metal 230 and Sn from the bump layer 215 orsolder layer 210. The diffusion barrier layer 225 may prevent CuSnintermetallic formation and whisker formation. The diffusion barrierlayer 225 may prevent bump delamination and improve the processing yieldof fabricating die package interconnects. The diffusion barrier layer225 may include, among others, any one of CoBP, CoWP, CoWB, CoWBP, NiBP,NiWP, NiWB, and NiWBP.

Electroless deposits may offer one or more advantages when deposited onirregularly shaped objects, patterns, and recesses. In electrolessplating, electrons are supplied by a chemical reducing agent. Ingeneral, electroless plating may refer to a reduction of metal ions froma solution containing a reducing agent. The reducing agent can supplyelectrons by oxidation on a catalytic surface. Electroless deposits mayhave high uniformity and little to no compressive stress during plating.Electroless deposits tend to be uniform in thickness over all of theshape of the underlying structure, therefore providing more uniformcurrent densities and reducing some electromigration issues. Electrolessbarriers may also offer the advantages of being low cost, selective, andamorphous.

The diffusion barrier may have other materials that may prevent orinhibit the diffusion of Sn with Cu through the diffusion barrier. Forexample, platable materials having slow reaction or diffusion with Snand Cu may be used, such as metals from group VIII (e.g., Co, Ni, Fe,Ru, Rh, Ir, and Os) alloying with Group VI (e.g., W, Mo, and Cr) andmetalloid (e.g., B, P, and N).

A wetting layer (not shown) may be placed on top to the diffusionbarrier layer 225. The wetting layer may also be selectively depositedon portions of the diffusion barrier layer 225. The wetting layer mayinclude any one of CoB, NiB, and NiP.

A bump layer 215 is placed on top of the wetting layer and a solderlayer 210 is placed on top of the bump layer 215. The die package 220 ison top of and electrically connected to the solder layer 210. Electricalconnection to the die package 220 may allow current to flow between thedie package and devices and interconnects near or on the substrate. Snmay be in the bump layer 215, the solder layer 210, or in both layers210, 215.

In one or more exemplary implementations in the present disclosure, Snmay be electroplated to suppress whisker formation and relatedelectromigration failures. The electroplating of Sn may also prevent lowtemperature (e.g., around 13.2° C.) phase transition of Sn and preventmechanical and electromigration failures related to beta Sn. Theelectroplating of Sn may include Sn and the alloys of Sn, such as 0.7Cu, Bi, Sb, and 3.5 Ag. Sn may be electroplated at a constant current(e.g., around 10-100 mA/cm2) or voltage from a solution containing Snsalt (e.g., Sn sulfate, Sn chloride), acid (e.g., sulfuric acid,sulfonic acid), and other additives (e.g., a suppressor, such aspolyether glycol or grain refiner and an anti-oxidant).

FIG. 3 is an exemplary flow diagram of the fabrication of the exemplaryimplementation shown in FIG. 2. One or more devices and Cu interconnectsmay be formed on the wafer at 310. The die package interconnect 200 maythen be passivated with SiN and polyimide at 312. By using lithographyand etching operations, a contact pad (not shown) may be opened for Cumetallization. The base layer metal 230 may be deposited using plasmavapor deposition (PVD), chemical vapor deposition (CVD), atomic layerdeposition (ALD), or plating at 314. The base layer metal 230 mayinclude an adhesion layer (e.g., Ti, TiN, or TiSiN) and a seed layer(e.g., Ni, NiV, or Co). An additional metal layer, such as Al, can beformed between the adhesion layer and the seed layer to improve barrierproperties of the base layer metal 230.

A photoresist layer may then be deposited and patterned at 316. Adiffusion barrier layer 225 can be formed at 318. The diffusion barrierlayer 225 may be electroless and may include any one of CoBP, CoWP,CoWB, CoWBP, NiBP, NiWP, NiWB, and NiWBP. Then, a wetting layer can bedeposited on the diffusion barrier layer 225 at 320. The wetting layermay include any one of CoB, NiB, CoP, and NiP. Electroplating of Sn oralloys of Sn can be performed at 322. Some alloys of Sn may include anyone of 0.7 Cu, Bi, Sb, and 3.5 Ag. The photoresist may then be removedat 324 and the base layer metal 230 may be etched at 326.

Forming the die package interconnect 200 with the diffusion barrierlayer 225 may entail using etching to pattern the base layer metal 230.The etching of the base layer metal 230 may reduce the degradation(e.g., corrosion or oxidation) of Sn bumps and polyimide.

FIG. 4 shows another exemplary implementation of the die packageinterconnect 400. The bump layer 415 is formed of Cu and is directly ontop of the base layer metal 430. The base layer metal 430 may be Cu. Thediffusion barrier layer 425 may be electroless and may be placed on topof the (Cu) bump layer 415 and below a layer of Sn or Sn alloy. Thediffusion barrier layer 425 may provide advantages similar to thediffusion barrier layer 225 of interconnect 200 of FIG. 2. For example,the diffusion barrier layer 425 may prevent Cu and Sn diffusion orintermixing between the (Cu) bump layer 415 and the Sn layer 435 bypreventing the diffusion of Cu and Sn through the diffusion barrier. Thediffusion barrier layer 425 may prevent whisker formation in the bumplayer 415. Although exemplary thicknesses of several layers 415, 425,435 are shown in the diagram, layer thicknesses can vary from what isshown.

A solder layer 410 may be formed above the Sn layer 435 and a packagelayer 420 may be connected to the solder layer 410. The package layer420 is electrically connected to all of the other conductive layers 410,435, 425, 415, 430 in the interconnect 400, allowing current to flowbetween the die package and devices and interconnects near or on thesubstrate.

FIG. 5 shows an exemplary flow diagram of the fabrication of theexemplary implementation shown in FIG. 4. The process flow forinterconnect 400 at 310, 312, 314, and 316 may occur in similar mannersand orders as the process flow for interconnect 200 in FIGS. 2-3. At518, a Cu bump layer 415 is formed and electroplated. Electroplating theCu bump layer 415 may provide the uniform thickness, whisker formationsuppression, and low to zero compressive stress advantages aselectroplating the Sn bump layer 215 in FIG. 2. The diffusion barrierlayer 425 may be electroless and may be formed on the bump layer 415 at520 and a wetting layer (not shown in FIG. 4) is formed on top of thebump layer 415 at 522. The electroless diffusion barrier layer 425 mayinclude any one of CoBP, CoWP, CoWB, CoWBP, NiBP, NiWP, NiWB, NiWBP, andthe wetting layer may include any one of CoB, NiB, CoP, and NiP.

At 524, the Sn layer 435 is formed and electroplated on top of thewetting layer. Electroplating the Sn layer 435 may provide similaradvantages as described above for electroplating Sn in interconnect 200.Such similar advantages may include suppression of whisker formation andpreventing low temperature phase transition of Sn. The photoresist maybe removed at 516 and the base layer metal 430 may be etched at 528. Asolder layer 410 may be formed above the Sn layer 435 and a packagelayer 420 may be connected to the solder layer 410. The solder layer 410may contain Sn and may also be electroplated.

FIG. 6 shows another exemplary implementation of the die packageinterconnect 600. The bump layer 615 may be formed of Cu and may bedirectly on top of the base layer metal 630. The base layer metal 630may be Cu. The diffusion barrier layer 625 may be electroless and may beplaced on top of a Cu bump layer 615 and below a layer of Sn or Snalloy. The diffusion barrier layer 625 may surround the bump layer 615,with the base layer metal 630 contacting a bottom surface of the bumplayer 615. All non-base layer metal 630 surfaces of the bump layer 615,including the top surface and sidewall surfaces, may be covered with theelectroless diffusion barrier layer 625. Hence, the outer surface of thebump layer 615 may be physically isolated from direct physical contactwith a layer that may include Sn. The electroless diffusion barrierlayer 625 may provide advantages similar to those of interconnect 400 ofFIG. 4. For example, the electroless diffusion barrier layer 625 mayprevent Cu and Sn diffusion or intermixing between Cu in the bump layer615 and Sn in a Sn layer 610, and may prevent whisker formation in thebump layer 615.

A solder layer 610 may be formed above the electroless diffusion barrierlayer 625 and a package layer 620 may be connected to the solder layer610. The solder layer 610 may include Sn and may be electroplated.

FIG. 7 shows an exemplary flow diagram of the fabrication of theexemplary implementation shown in FIG. 6. The processing flow forinterconnect 600 at 310, 312, 314, 316 and 518 may occur in similarmanners and orders as the processing flow for interconnect 400 in FIGS.4-5. At 720, the photoresist may be removed and the base layer metal 630may be etched at 722. The diffusion barrier layer 625 may be electrolessand may be formed on the bump layer 615 at 724 and a wetting layer (notshown in FIG. 6) may be formed on top of the bump layer 615 at 726. Theelectroless diffusion barrier layer 625 may include any one of CoBP,CoWP, CoWB, CoWBP, NiBP, NiWP, NiWB, NiWBP, and the wetting layer mayinclude any one of CoB, NiB, CoP, and NiP. Other conductive layers, suchas the solder layer 610, may be formed on top of the electrolessdiffusion barrier layer 625 and the wetting layer, and can be contactedto a package layer 620.

FIG. 8 shows an exemplary implementation of the die package structure inan electrical computer system. One or more interconnects and layers fora die package are formed on a substrate, as described above with respectto FIGS. 3, 5, and 7. When placed on a circuit board 850, the diepackage 810 can connect the internal circuitry within the die packagewith circuitry that is external to the die package and on the circuitboard 850. The circuit board 850 may have other chips and components,such as a memory 843, a central processing unit (CPU) 825, and acontroller or some other logic unit 833. The circuit board 850 may bemade with multiple layers for routing signals between components, andmay be used in a system of computers and/or electronics 860.

A number of implementations of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, the processing order may vary from the processing order shownin FIGS. 3, 5, and 7. In FIG. 7, for instance, after the Cu layer hasbeen formed and electroplated 518 for interconnect 600, the electrolessdiffusion barrier layer 625 may be deposited 724. Following theformation 726 of the wetting layer on the electroless diffusion barrierlayer 625, the photoresist may be removed 720 and the base layer metal630 may be etched 722. In another example, an electroless barrier can beused to prevent the intermixing of other metals besides Cu and Sn, suchas preventing the intermixing of Au and Al.

What is claimed is:
 1. A method of forming an assembly, comprising:providing a die, the die having a metal pad including aluminum, a baselayer metal (BLM) disposed on the metal pad, the BLM including titanium,a bump disposed on the BLM, the bump including copper, and a firstsolder layer disposed on the bump, the first solder layer including tin;providing a package, the package having a first side and an opposingsecond side, and a second solder layer disposed on the first side of thepackage, the second solder layer including tin; connecting the secondsolder layer of the package to the first solder layer of the die toenable electrical current to flow between the package and the die. 2.The method of claim 1, wherein the bump is disposed directly on the BLM.3. The method of claim 1, wherein the base layer metal (BLM) furtherincludes copper.
 4. The method of claim 1, wherein the first solderlayer further includes silver.
 5. The method of claim 1, wherein thesecond solder layer on the package further includes silver and copper.6. The method of claim 1, wherein the die further comprises a diffusionbarrier layer disposed between the bump and the first solder layer. 7.The method of claim 6, wherein the die further comprises a wetting layerdisposed between the diffusion barrier layer and the first solder layer.8. The method of claim 6, wherein the diffusion barrier layer includesphosphorous.
 9. The method of claim 1, wherein the assembly issubstantially free of lead.
 10. The method of claim 1, wherein one ofthe first solder layer and the second solder layer comprises an elementnot included in the other of the first solder layer and the secondsolder layer.
 11. A method of forming an assembly, comprising: forming ametal pad on a die, the metal pad including aluminum; forming a baselayer metal (BLM) on the metal pad, the BLM including titanium; forminga bump on the BLM, the bump including copper; forming a first solderlayer on the bump, the first solder layer including tin; forming asecond solder layer on a side of a package, the second solder layerincluding tin; and subsequent to forming the metal pad, the BLM, thebump, the first solder layer and the second solder layer, connecting thesecond solder layer of the package to the first solder layer of the dieto enable electrical current to flow between the package and the die.12. The method of claim 11, wherein the bump is formed directly on theBLM.
 13. The method of claim 11, wherein the base layer metal (BLM)further includes copper.
 14. The method of claim 11, wherein the firstsolder layer further includes silver.
 15. The method of claim 11,wherein the second solder layer formed on the package further includessilver and copper.
 16. The method of claim 11, further comprising:forming a diffusion barrier layer on the bump, wherein the first solderlayer is formed on the diffusion barrier layer.
 17. The method of claim16, further comprising: forming a wetting layer on the diffusion barrierlayer, wherein the first solder layer is formed on the wetting layer.18. The method of claim 16, wherein the diffusion barrier layer includesphosphorous.
 19. The method of claim 11, wherein the assembly issubstantially free of lead.
 20. The method of claim 11, wherein one ofthe first solder layer and the second solder layer comprises an elementnot included in the other of the first solder layer and the secondsolder layer.